Cytopathologic Diagnosis of CSF Metastasis: A 10-year Institutional Review

Summary: Introduction: Leptomeningeal metastasis is a rare and poor prognostic factor in patients with solid malignancies of different origins. CSF cytology has been shown to be a reliable method in diagnosing CSF metastasis. This study aims to evaluate the reliability of CSF cytology for detection and subclassification of CSF metastasis and to report the different types of solid malignant CSF metastasis at our institution. Materials and Methods: The laboratory information system was queried over a period of 10 years (2014-2024) to identify all CSF cytology cases with metastatic disease. Only metastatic solid malignancies were included in our review. Hematologic malignancies and cases diagnosed as "Atypical" or "Suspicious for malignancy" were excluded. Results: A total of 286 specimens from 171 patients were identified. The 171 patient cases were diagnosed as follows: 51 metastatic breast carcinoma (ductal and/or lobular), 29 metastatic lung adenocarcinoma, 16 metastatic melanoma, 5 metastatic urothelial carcinoma, 4 metastatic serous carcinoma (endometrial or ovarian), 3 metastatic gastric adenocarcinoma, 3 esophageal adenocarcinoma, 2 metastatic GE junction adenocarcinoma, 2 metastatic cholangiocarcinoma, 2 metastatic small cell carcinoma, 1 metastatic pulmonary large cell neuroendocrine carcinoma, 1 metastatic pancreatic adenocarcinoma, 1 metastatic prostate carcinoma, 1 metastatic bladder adenocarcinoma with signet ring features and 1 metastatic NUT carcinoma of the orbit. Additionally, there was 31 (18.1%) metastatic carcinomas not otherwise specified (NOS) and 18 (10.5%) metastatic adenocarcinomas NOS whereby the limited specimen cellularity and inability to perform additional ancillary testing precluded further subclassification. Conclusions: Cytology can provide a reliable and accurate method for diagnosing solid malignant CSF metastasis. Similar to the literature the most common tumor origins at our institution were breast, lung and melanoma. While urothelial and mullerian CSF metastasis appear to be overrepresented at our institution, gastrointestinal origin, pancreatobiliary primaries, and prostatic origin were uncommon. Although further subclassification could not be performed on 28.6% of cases at our institution, rare, underreported tumor origins were detected including metastatic bladder adenocarcinoma with signet ring features and metastatic NUT carcinoma of the orbit.